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Author Topic: periodic hum in soundtrack Joe Beres Jedi Master Film Handler Posts: 606 From: Minneapolis, MN, USA Registered: Nov 2000 posted 09-18-2001 08:38 PM                     Hey there, I have a weird hum that fades in and out of the sound from one of the projectors in a changeover system. I have Dolby reverse scan led heads running into a newly installed Dolby CP 650. The hum didn't seem to appear instantly with the new processor, but has reared its very ugly head in the last few shows. It fades in then fades out. It is usually present for only a few seconds then fades out for inconsistent amounts of time. ANy ideas on what this might be? Any help or advice would be much appreciated. Thanks, Joe Beres  |  IP: Logged Aaron Sisemore Flaming Ribs beat Reeses Peanut Butter Cups any day! Posts: 3061 From: Rockwall TX USA Registered: Sep 1999 posted 09-18-2001 08:54 PM                     What frequency is the hum? 60Hz? 90Hz? 120Hz? or is it more of a 'growling' or 'rumbling' sound (almost always caused by the optical reader picking up DTS timecode) Aaron  |  IP: Logged Paul G. Thompson The Weenie Man Posts: 4718 From: Mount Vernon WA USA Registered: Nov 2000 posted 09-18-2001 08:57 PM                     Sounds like a lateral guide roller going askew, if you are running the analog soundtrack.  |  IP: Logged Joe Beres Jedi Master Film Handler Posts: 606 From: Minneapolis, MN, USA Registered: Nov 2000 posted 09-18-2001 08:59 PM                     by ear, i would say its around 60Hz, but i haven't had a chance to run too many tests yet. the screening room doubles as a classroom during the day. i'm hoping to get a bit of advice in hopes of having more information to go on when i get a chance to check into things a bit further.  |  IP: Logged Rick Long Jr Expert Film Handler Posts: 211 From: Toronto, Canada Registered: Jul 2000 posted 09-21-2001 09:50 PM                     Stray light hitting the cell can cause hum, but it is unlikely with reverse scan readers. You didn't mention if it is there only with film running or not.If not is it present in other formats? If it is only there with film running, is it on both machines? You might as well check the easy stuff first and run some Buzz Track and confirm the lateral alignment on both machines, as this can definitely cause a weird noise that fades in and out.  |  IP: Logged Steve Scott Phenomenal Film Handler Posts: 1300 From: Minneapolis, MN Registered: Sep 2000 posted 09-22-2001 12:10 AM                     It could be bad wiring in an amplifier or speaker. I'd run the Buzz track first to see if the soundhead is properly alligned, but check the connections of all of your speaker posts/plugs too. It might be some interfearance from the projector motor, but I'm thinking more along the lines of a B chain problem.  |  IP: Logged Christopher Duvall Jedi Master Film Handler Posts: 500 From: Denver, CO Registered: Dec 1999 posted 09-22-2001 01:30 AM                     Do you have flourescent bulbs in your booth? If you do, they "strobe" at about 60 times a second. That could account for the 60Hz hum you are hearing. Just a thought...  |  IP: Logged Paul G. Thompson The Weenie Man Posts: 4718 From: Mount Vernon WA USA Registered: Nov 2000 posted 09-22-2001 01:34 AM                     see if you can establish a pattern. Could be an electrical load unbalance, and you could see a ground loop hum because of the load shifting for one reason or another.  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 09-22-2001 09:55 AM                     Flourecnet light hum will be 120 hz as the lamps switch on and off on each zero crossing produceing 120 pulse per second (one for each half wave). That makes a easy way to tell if it is induced or ground loop that will be 60 Hz or room light which will be 120 Hz Perfs being scanned is 96 Hz  |  IP: Logged Joe Beres Jedi Master Film Handler Posts: 606 From: Minneapolis, MN, USA Registered: Nov 2000 posted 09-22-2001 12:08 PM                     thanks for all of your ehlp. it turns out that paul was right. some minor adjustments to the lateral guide roller solved the problem. joe  |  IP: Logged

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